The present invention relates to the field of three-dimensional image capture and in particular to a modulated illumination source used in the capture of image depth information with a scannerless range imaging system.
Distance (or depth) information from a camera to objects in a scene can be obtained by using a scannerless range imaging system having a modulated illumination source and a modulated image receiver. In a method and apparatus described in U.S. Pat. No. 4,935,616, a scannerless range imaging system uses an amplitude-modulated high-power laser diode to simultaneously illuminate a target area. Conventional optics confine the target beam and image the target onto a receiver, which includes an integrating detector array sensor having hundreds of elements in each dimension. The range to a target is determined by measuring the phase shift of the reflected light from the target relative to the amplitude-modulated carrier phase of the transmitted light. To make this measurement, the gain of an image intensifier (in particular, a micro-channel plate) within the receiver is modulated at the same frequency as the transmitter, so the amount of light reaching the sensor (a charge-coupled device) is a function of the range-dependent phase difference. A second image is then taken without receiver or transmitter modulation and is used to eliminate non-range-carrying intensity information. Both captured images are registered spatially, and a digital processor is used to extract range data from these two frames. Consequently, the range associated with each pixel is essentially measured simultaneously across the whole scene.
A scannerless ranging system, such as the system described above, typically uses a laser for field illumination in order to capture depth information. Lasers are capable of high power and very high frequency modulation. However, a primary concern is eye safety, which requires proper safety glasses or alternatively requires the laser design to provide added protective measures, e.g., output radiant power limits, safe viewing distance, etc. To achieve sufficient powers of illumination for image capture, scannerless ranging systems use multimode lasers. Due to their nature, multimode laser diodes typically have significant spatial structure along with astigmatism, speckle and large width-to-height ratios (ellipticity) in the beam. As a result, large non-uniform regions can appear when using the beam to illuminate an object field. The spatial structure can also shift with varying temperatures and drive current. To overcome these effects, optical components, e.g., an anamorphic prism, are placed in the path of the illumination source to shape the beam and reduce ellipticity. Diffuser plates are also placed into the beam path to reduce spatial structure in the illumination, to provide beam spreading, and to comply with requirements for eye safety. Other optical components could also be used in lieu of a diffuser to produce beam uniformity and spreading. For long distances, the coherency property of a laser provides the advantage of maintaining low dispersion.
What is needed is an alternative to the use of laser diodes for depth capture to overcome some of the disadvantages presented by the use of a laser for field illumination, namely: the concern for eye safety, beam structure, speckle, additional optics and cost. Moreover, in the case of field illumination for closer range image capture (e.g., 40 feet or less), the coherency property of a laser is a disadvantage since beam spreading and good uniformity are desired, thus necessitating further optics. In consideration of power density, a single laser component does provide higher output power density (compared, e.g., to a single LED), however high current thresholds must be overcome to drive the laser.
As described in the Sandia Lab News (vol. 46, No. 19, Sep. 16, 1994), the scannerless range imaging system described in the ""616 patent may alternatively use an array of amplitude-modulated light emitting diodes (LEDs) to completely illuminate a target scene. Eye exposure to an LED source illumination can be tolerated, as one might expect in a picture-taking scenario where eye sensitivity is present. However, the design for the modulated LED source poses a challenge, particularly as to the scalability and reliability of the design, as well as to operation at the required high modulating frequencies.
It is an object of the invention to provide a scannerless range imaging system that is capable of providing reliable illumination for closer range image capture.
A further objective of the invention is to provide improved eye safety in a scannerless range imaging system, particularly for closer range image capture.
A further object of the invention is to provide easily scalable, high frequency drive circuitry for an LED illumination source for a scannerless range imaging system.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, a light emitting diode (LED) illumination device for generating modulated light for a scannerless range imaging system includes circuitry for generating a drive signal having a given phase and frequency characteristic; a plurality of light emitting diodes arranged in a plurality of LED banks, wherein each LED bank comprises a plurality of light emitting diodes connected in series to the circuitry to receive the drive signal; and a switching stage for simultaneously activating the plurality of LED banks such that the LED banks generate the modulated light according to the phase and frequency characteristics of the drive signal.
This invention provides an alternative to the use of laser diodes for depth capture, thereby overcoming some of the disadvantages presented by the use of a laser for field illumination, namely: eye safety, beam structure, speckle, high threshold currents, additional optics and cost.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.